It's tied to the approved Rules and Regulations of the BSA, Article VIII, Section 3:
Clause 1: All individuals in unit leadership positions (including Lone Cub Scout and Boy Scout friend and counselors) shall be issued commissions by the Boy Scouts of America
Clause 3: Commissioned officials must be at least 21 years of age, with the exception of the individuals at least 18 years of age who may be commissioned as assistant unit leaders. (Associate Venturing Advisors must be 21 years of age.)
I'm not sure why it's written that way, but Eagle makes a good point, but it could very well be an insurance issues as well.